The present invention relates to compositions suitable for colouring ceramic manufactured articles and to the relevant colouring process.
In particular, the compositions of the invention consist of water solutions or solutions of water and water-miscible solvents, of gold organic complexes compatible with other colouring cations optionally present in the solution releasing low quantities of corrosive vapours during the firing cycle.
Said solutions allow the obtainment of ceramic manufactured articles in shades from pink to purple to violet after a firing cycle ranging from 750xc2x0 C. to 1,300xc2x0 C.
The use of coloured ceramic manufactured articles as well as the compositions and process adopted to obtain the relevant colours have been known since long. One of the methods most commonly used consists in the addition of powdered pigments, in particular inorganic oxides and mineral colouring matters, to the ceramic mixture (vitrified stoneware) before firing. The ceramic manufactured article is thus coloured through its whole thickness, although with large consumption of colouring matter, which is the most expensive component.
According to a process used, the surface of the ceramic material is caused to absorb, either after partial firing (as disclosed e.g. in German patent 2,012,304) or simply after moulding and before firing (as disclosed e.g. in Swiss patent 575,894), a water solution of inorganic salts or metal complexes (as disclosed e.g. in Sprechsal, vol. 119, No. 10, 1986, in EP 0704411 and in patent PCT, WO 97/38952), which become stable colours at high temperature during the ceramic firing cycle.
The water solution is applied to the ceramic material before final firing. This process is particularly advantageous because it allows the colouring of very thin layers: therefore, it is widely used for flat manufactured articles (such as e.g. floor and wall tiles).
Another problem to be solved when using colours in a water solution is the obtainable depth of colour penetration into the ceramic material. In fact, it was experimentally found that the depth of penetration depends on several parameters, such as the viscosity and surface tension of the colouring solution, the application temperature, the quantity of water optionally sprayed on the manufactured article once the colouring solution has been applied. The water solution is applied to the ceramic manufactured article by immersion, spraying, disk, and silk-screen techniques.
Of cardinal importance is the application technique: in particular, the quantity of colouring solution that may be applied by disk and spraying techniques is as high as 400 to 600 g/m2; by silk-screen type techniques it usually amounts to 100 to 200 g/m2 and sometimes even to 400 g/m2, when thickened screens made of a small number of threads are used.
Silk-screen type techniques are very much in demand, being the only techniques allowing graphic decorations and drawings, which otherwise cannot be obtained, and requiring lower quantities of colouring matter. When said techniques are used, the colouring solutions are to be thickened with appropriate thickening agents, e.g. modified glucomannans, starch and modified starch derivatives, cellulose and modified cellulose derivatives, or other polymeric substances, soluble or dispersible in a water solution.
Colour penetration into the ceramic material before firing can be obtained by spraying relatively high quantities of water on the manufactured article after application of the colouring solution. However, the resulting colours are less intense than those obtained using other techniques.
Colour penetration into the material is particularly important in the case of xe2x80x9csmoothedxe2x80x9d vitrified stoneware tiles.
The term xe2x80x9csmoothedxe2x80x9d means that the vitrified stoneware surface has been abraded with diamond wheels by 0.8 to 1.5 mm and subsequently smoothed or polished with appropriate felt until obtaining a glassy surface.
It follows that colour penetration into stoneware articles to be smoothed after firing must reach a depth of 1.6 mm min.
Other methods have been developed for the smoothing of very thin (1 to 10 0) surface layers of the manufactured article.
The use of water solutions of gold compounds for the colouring also in-depth of ceramic manufactured articles is well known. The compounds disclosed in German patent 4,320,072 for said application substantially are gold chlorides also reported in the preceding literature (cf. xe2x80x9cEncyclopedia der technischen Chemiexe2x80x9d, F. Ullmann, 1929, vol. 4, pp. 837-838). However, the gold chloride solution suffers from the inconvenience of being strongly acid, as it contains hydrochloric acid. In the absence of excess hydrochloric acid, the solution is unstable and the gold compound hydrolyses easily with formation of insoluble compounds.
It follows that the solution is corrosive and impairs the apparatus used. In particular, in the case of silk-screen type technique, it rapidly impairs the printing screen.
WO 97/21646 discloses the use of gold sodium thiosulphate solutions, Na3Au(S2O3)2, stabilised with sodium sulphite, for ceramic surfaces colouring by water solution absorption.
From the compositions of solutions 1, 2, and 3 described therein, it is possible to calculate that 4.7 g SO2 or 5.88 g SO3, or an intermediate value in the case of mixtures thereof is released per g Au deposited on the surface.
In both cases (use of gold chloride and gold thiosulphate), high amounts of strongly corrosive vapours rapidly impairing the heater metal structures, are released. Therefore, vapours are to be abated to prevent the emission of same into the environment.
The use of precious metals water solutions in high concentrations, in the form of thiol derivatives, to obtain thin metal films for decorative purposes, e.g. for dishes, or for electronic purposes, e.g. for printed circuits, is already known.
The following are examples of said use:
1. U.S. Pat. No. 5,545,452 discloses the use of Au thiolates water solutions with a metal content of 2 to 25% by wt. (column 3, rows 10 to 20) to obtain thin metal films for decorative purposes (column 1, rows 38 to 40);
2. EP No. 514,073, like the U.S. patent above, discloses the use of thiolates water solutions to obtain thin films made of Au or other precious metals to decorate the outer surface, and not the inside, of baked ceramic manufactured articles.
Considering that it is very simple to colour ceramic materials by disk, spraying and silk-screen techniques, the ceramic industry is highly interested in the possibility of using colouring water solutions based on gold to be applied by said techniques, and offering the advantage of
releasing the lowest possible quantity of very noxious or corrosive vapours in the heater;
being compatible with water solutions of organic derivatives of other cations used for the superficial and in-depth colouring of ceramic manufactured articles.
Therefore, it is an object of the present invention to provide colouring formulations in the form of water solutions, which
release low quantities of corrosive vapours (xe2x89xa62 g SO2/g Au deposited);
are compatible with water solutions of organic derivatives of other cations used for the superficial and in-depth colouring of ceramic manufactured articles;
colour the ceramic manufactured articles at their surface and to a depth of at least 1 mm.
The Applicant, who has full-fledged experience in the production and sale of colouring matters for ceramic tiles, has now found that water solutions or water mixtures with alcohols or other water-miscible organic solvents, of monovalent gold organic derivatives, can be used to obtain-after firing-colour shades varying from pink to purple to violet on manufactured articles consisting of a conventional ceramic mixture.
The water or hydroalcoholic solutions being an object of the present invention are particularly useful for colouring tiles of vitrified stoneware, either at their surface or to a depth of 1 to 3 mm from their surface.
In-depth colouring is essential for maintaining the manufactured article decoration after smoothing.
It is, therefore, a fundamental feature of the present invention to use water solutions or water mixtures with hydrophilic organic solvents, of gold organic complexes, which are compatible with derivatives of other cations and release low quantities of corrosive vapours (xe2x89xa62 g SO2/g Au deposited).
Said solutions are used to treat ceramic articles before firing. This makes it possible, after firing, to obtain pink, purple and violet coloured articles, when solutions contain gold only, or new colour shades when solutions are mixed with derivatives of one or several of the following cations: Fe, Cr, Co, Mn, Cu, Ru, Pd, Zr, V, Ni, Sb, W, Zn, Sn.
The gold compounds that may be used according to the present invention belong to the class of monovalent gold thiolates, wherein Au is bound to an S atom, and are substantially represented by the following general formulas:
Auxe2x80x94Sxe2x80x94Rxe2x80x94X and Auxe2x80x94Sxe2x80x94Rxe2x80x94H
where R stands for a linear or branched bivalent radical of aliphatic or aromatic or cycloaliphatic or heterocyclic type, optionally with substituents, such as for example aminic, amidic, hydroxylic, carboxylic, hydrocarbylic or carbonylic groups or CONHxe2x80x94, in the chain;
X stands for a monovalent group selected out of xe2x80x94COOH, SO2OH, xe2x80x94OH, xe2x80x94CONH2, xe2x80x94NH2; xe2x80x94Oxe2x80x94P(O)(OH)2, in which H atoms may be replaced by alkyl groups and wherein acid groups may be salified with amines or alkaline or alkaline earth metals and basic groups may optionally be salified with organic acids.
In particular, monovalent Au thiolates to be used according to the present. invention may be in the form of alkaline, alkaline earth metal salts or of variously substituted amines.
The water solutions of Au compounds according to the invention are stable, substantially neutral and release a quantity of corrosive vapours of 2 g SO2 max./g Au deposited on the manufactured article surface to be coloured.
Said solutions are used in quantities corresponding to 0.1 to 20 g Au (as element) per m2 of surface to be coloured.
By way of example, the gold thiolates that may be used according to the invention derive from:
(N) acetylcysteine: 
4-mercaptopyridine: 
2-mercaptoacetyl-glycine:Hxe2x80x94Sxe2x80x94CH2xe2x80x94COxe2x80x94NHxe2x80x94CH2COOH
2-mercaptopropionyl-glycine: 
(d,I) mercaptosuccinic acid: 
4,6-dihydroxy-2-mercapto-pyrimidine: 
2-mercapto benzimidazole: 
cysteine: 
2-mercaptopropionic acid: 
The compatibility of the gold thiolates according to the invention with the ceramic colouring systems based on metallic compounds (in particular of Fe, Ni, Cr, Co, Sn, Mn, Cu, Ru, Pd, Zr, V, Sb, W, Zn, Sn) in aqueous solutions has been ascertained through stability tests of the aqueous solutions containing beside Au thiolate also one or more compounds of the above mentioned metals and also through ceramic coloration tests by the same aqueous solutions after ageing. Unforeseable very valuable colours were obtained.
Gold thiolates to be used according to the invention may generally be prepared by causing an Au(III), in the form of a tetrachloroauric acid, to react in water solution with a thioether S(Rxe2x80x2)2, giving the reduction of Au(III) to Au(I), and then with the desired thiol HSRxe2x80x3 (Rxe2x80x3=xe2x80x94RX or xe2x80x94RH as per the general formulas shown above):
This method, described in Inorganic Synthesis, 23 (1985) pp. 191-195, is based on some reactions reported therein.
A typical process for applying colouring compositions according to the invention consists in the following steps:
a) drying at 100xc2x0 C. of the article to be coloured to a water residue of 0.5% by wt. max.;
a.1) pre-treatment, if any, of the dried article with water up to a max. quantity of 300 g/m2 manufactured article;
b) treatment of the pre-treated article with a water solution of the colouring composition in a quantity of 30 to 600 g/m2 of the final coloured surface;
b.1) post-treatment, if any, of the treated article with water up to a max. quantity of absorbed water of 300 g/m2 ceramic manufactured article;
c) equalisation of the post-treated article at room temperature for 8 hours to homogenise the solution absorption;
d) oven firing according to the usual ceramic cycle at a temperature of 1,000 to 1,300xc2x0 C.
The concentration of Au thiolate solution to be used according to the invention generally ranges from 0.1 to 2% Au (expressed as element).
Some examples of the colours obtained by experimental runs are shown in the Table 1 reported hereinafter.
All runs were carried out on the basis of the following process:
1) drying at 1000xc2x0 C. of two 33xc3x9733 cm supports made of mixture A to a water residue of 0.5% by wt. max.;
2) supports cooling to room temperature;
3) deposition of 0.4 g of each solution on 10 cm2 of surface of each support;
4) supports equalisation at room temperature for 2 hrs and for additional 2 hrs in a thermoventilated oven at 600xc2x0 C. to homogenise the solutions absorption;
5) oven firing according to the usual ceramic cycle;
6) removal of a surface layer (0.8-0.9 mm) and smoothing;
7) colour detection on non-smoothed and smoothed support;
8) non-smoothed support cutting and penetration depth detection.
The composition of the ceramic mixture used is as follows (% by wt.) SiO2 64.4%; Al2O3 21.8%; K2O 3.8%; Na2O 0.8%; CaO 0.6%; MgO 0.1%; TiO2 0.3%; Fe2O3 0.2%; ZrSiO4 5%; H2O to 100%.
Colours are as per the Colour Atlas.